Does anyone happen to know how many [very roughly] genera have a partially separated heart chamber or a partially developed septum? If you compared the number of organisms with (say) 3 or 4- chambered heart with the number of "intermediate" ones with only partially separated chamber, what would be the ratio between them [again very roughly]?

Well first off, most animals have chambered hearts. Humans have 4 chambers which is divided by the septum in the middle and two valves. But I think like reptiles and such have 3 chambered hearts. So basically to find your answer we have to count all the (did you mean genus?) types that have 3 versus those that have 4.

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Class Cyclostomata(vertebrates without jaws) have to heart chambers-1 atrium(or auricle) and 1ventricle. Fish also have the same kind of heart-2 chambers. Amphibians have a 3 chambered heart: 2 atriums and 1 ventricle. Reptiles have 2 atriums and 1 ventricule that has a septum that separates it incompletly into two chambers. At crocodiles the separation is complete and the heart becomes "with 4 chambers". Birds and mamals also have a heart with 4 chambers

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